(1) Field of the Invention
This invention relates to a novel process for producing salts of pyruvic acid by the oxidation of salts of lactic acid with an oxygen-containing gas.
(2) Description of the Prior Art
Pyruvic acid and salts thereof are important intermediates found in the pathway of carbohydrate metabolism within the living body. Industrially, these compounds are useful as the starting materials for the production of L-tryptophan by a fermentation process in which indole, pyruvate, and ammonia are reacted by the action of tryptophanase. In addition, they have great utility in the production of L-cysteine by a fermentation process using pyruvate, ammonia, and hydrogen sulfide as starting materials and in the production of L-DOPA by a fermentation process using catechol, pyruvate, and ammonia as starting materials.
In the prior art, a number of processes for producing pyruvic acid and salts thereof have been proposed. They include, for example, (1) a process in which sodium cyanide and acetyl chloride are reacted to form acetyl cyanide and the cyanide is then hydrolyzed; (2) a process in which tartaric acid is dry-distilled in the presence of potassium hydrogensulfate; (3) a fermentation process using lactic acid as starting material; (4) a process in which lactic esters are oxidized, for example, with potassium permanganate; (5) a process in which propylene glycol is oxidized; and the like.
However, the above-described processes have various drawbacks. Specifically, the process (1) is disadvantageous in that the starting materials are expensive, the yield of the desired product is low, and the formation of considerable amounts of by-products makes it difficult to separate and purify the desired product. In the process (2), the starting material (tartaric acid) is expensive, the subsidiary material (potassium hydrogensulfate) is consumed in large amounts, and the yield is not so high as would be desirable. In the process (3), .alpha.-ketoglutaric acid is inevitably formed as a by-product. The process (4), in which lactic esters are oxidized with a suitable agent such as potassium permanganate, has the disadvantages that the potassium permanganate is consumed in an amount greater than its chemical equivalent and that the recovery and re-oxidation of manganese require extremely complicated procedures. The process (5) based on the oxidation of propylene glycol is not suitable for the selective production of pyruvates because by-products such as lactates are formed in considerable amounts.